Colored lustrous pigments are known in which the lustrous pigment part was natural pearlescent materials or synthetic pearlescent substances (also called nacreous pigments) and the colors were a wide variety of inorganic and organic coloring agents or dyestuffs. The processes used to make colored lustrous pigments in prior art encountered numerous difficulties among which the following are noteworthy: severe bleeding of the color on filtration of the coated product from the suspension; poor adherence of the dyestuff on the surface of the pigment such that the color could be washed off with water; difficulty of retaining luster with increased color intensity; and nonuniform distribution of the dyestuff on the pigment surface.
The foregoing problems are in part described in U.S. Pat. No. 4,084,983. This patent relates to the use of titanium dioxide-coated-mica pigments to produce colors due to interference phenomenon and additional color effects achieved by coating organic dyes on the surface of these pigments. In an attempt to overcome the problems encountered in the art, the dyestuff is chemically bound on the surface of the pigment with the help of a laking reagent. For example, one laking reagent used was aluminum chloride which on hydrolysis produced layers of aluminum hydroxide with which the dye reacted to form the insoluble color lake thereby permitting it to deposit on the surface of the pigment. Although this approach particularly addresses the problem of poor adherence of the organic dye on the surface of the pigment, the problem of heavy bleeding of the uncoated dye remains severe. The failure of the previous art to control this problem was a serious obstacle in developing a suitable process of preparing colored lustrous pigments of superior quality. This failure, as will be shown later, was the consequence of the inefficiency of the laking process, which resulted from the manner in which the laking was carried out and from the conditions maintained in the coating of the dye on the surface of the pigment.
There are important differences between the instant invention and U.S. Pat. No. 4,084,983. The latter employs a procedure in which the aluminum hydroxide is first deposited upon the platey pigment surface. In the instant invention, the reverse procedure is followed, i.e., the soluble dye is allowed to adsorb first on the platey pigment surface, followed by the addition of a soluble aluminum or zirconium compound to form the corresponding hydroxide precipitate, which completes the laking reaction. This method provides for an important improvement compared to that of U.S. Pat. No. 4,084,983. The latter suffers from heavy bleeding of the soluble dye whereas the present approach eliminates bleeding almost completely, which has the advantages that none of the dye material is wasted and that the soluble dye does not contaminate other equipment. A further important advantage is that it does not present a waste-treatment problem.
The second difference is that the laking reaction can be carried at relatively higher concentrations of reagents in contrast to the procedure of U.S. Pat. No. 4,084,983. For example, the aluminum or zirconium chloride reaqent, which is hydrolized, is used at a concentration of 20-30% instead of 2.4-5%. The reaction is carried out with pigment concentration of 20% w/v as compared to 2% in the procedure of U.S. Pat. No. 4,084,983. The combination of the substantially higher concentrations along with the reverse mode of addition of reagents helps to prevent dye bleeding. More significantly, and unexpectedly, the high concentration procedure leads to superior products.
The third notable difference is that in the instant invention, the laking reaction can be carried out at ambient (ca. 25.degree. C.) temperature, whereas in U.S. Pat. No. 4,084,983 high temperatures in the range of 60.degree.-90.degree. C. are used for both the deposition of aluminum hydroxide and the precipitation of the dye. The higher temperature produces further bleeding and lowers the quality of the product. Laboratory experiments carried out to follow U.S. Pat. No. 4,085,983 resulted in severe bleeding of the dye and yielded products with poor reflectivity and evidence of agglomeration.
Finally, a significant contribution of the present invention is that it is not limited to the production of aluminum lakes. It has been found that lakes made from zirconium chloride are of comparable quality to those made from aluminum salts. Moreover, a lesser quantity of the zirconium chloride is necessary to carry out the laking process, because the tetravalent zirconium is more effective in precipitating the dyes. An additional advantage is that the precipitated zirconium lakes can be filtered more rapidly, making the process more efficient. As will be shown later, salts of other metals like calcium, barium, and strontium can also be used as laking reagents.
The main object of this invention is to develop an efficient laking process which is practical and economical for preparing dye-coated pigments with strong color intensity and superior luster. More specifically, one object, among others, of this invention is to provide a method of coating organic dyes on nacreous pigments whereby not only heavy bleeding is prevented but preferably even slight bleeding is not permitted in order that no trace of dye is found in the waste water system. This and other objects of the invention will become apparent to those skilled in the art from the following detailed description.